Permanent-magnet-type rotating electrical machines utilizing permanent magnets that do not require supply of magnetic field energy from outside are widely used in order to meet the demand for smaller sizes and higher outputs in industrial rotating electrical machines and in automotive rotating electrical machines that are used, for instance, in electric vehicles and hybrid automobiles.
The structure of the stator windings of a permanent-magnet-type rotating electrical machine is roughly divided into concentrated winding, in which a given coil is wound around one tooth, and distributed winding in which a given coil is wound across a plurality of teeth. Herein coil end length is shorter in concentrated winding than in distributed winding, and hence the axial length of the rotating electric machine can be shortened.
On the other hand, the magnetomotive force generated by the stator windings with concentrated winding includes low-order harmonic components that do not contribute to torque. These harmonic components give rise to the problem of increases in torque ripples, and occurrence of electromagnetic excitation force of low-order deformation mode. At specific revolutions at which the electromagnetic excitation force matches the resonant frequencies of parts of the rotating electrical machine such as a stator or frame, the electromagnetic excitation force resonates with these parts, and noise occurs as a result.
In order to reduce torque ripples, therefore, a permanent-magnet-type rotating electrical machine has been proposed (for instance PTL 1) that is provided with a plurality of types of steel plates stacked in the axial direction, the plurality of types of steel plates being disposed in such a manner that the central positions of tooth tips of the stator in the circumferential direction are offset, in the circumferential direction, from the center of a tooth body.
In order to reduce the electromagnetic excitation force a method has also been proposed (for instance, NPL 1) that involves calculating an appropriate negative d-axis (flux axis) current for which the electromagnetic excitation force is minimized, when stator windings are energized with a negative d-axis current.